Hundreds of patents have been granted for power tongs designed to make up and break out oilfield tubulars. Most commercially successful power tongs have an open throat so that the power tong can be moved laterally on or off the tubular. Power tongs with closed throats have utility for certain applications, but generally are not as widely used as rig power tongs for connecting and disconnecting tubulars as they are run into and out of the well.
Chrome plated tubulars are being increasingly popular due to the increased protection provided by highly corrosive downhole environments. The advantage of plating the tubular is substantially sacrificed, however, if the tong dies which make up the connection “mark” the OD of the tubular to the extent that the chrome plating is pierced. When the tubular is then placed downhole, corrosive fluids may act on this damaged area, thereby corroding the steel tubular, and may spread as the corrosive fluids get under the chrome layer.
In an attempt to minimize marking of chrome plated tubulars, some power tongs have employed aluminum dies. Aluminum substantially reduces markings since the aluminum material is softer than both the chrome and the oilfield tubular. The problem with this practice is that the aluminum dies, which reduce marking by providing relatively smooth biting surface, do not adequately grip the tubular to perform the makeup or breakout operation. Accordingly, aluminum dies in power tongs commonly slide on the tubular surface during the makeup or breakout operation, which then further scrapes and damages the chrome plating on the oilfield tubular.
U.S. Pat. No. 4,084,453 discloses a power tong with a rotary cage plate and a rotary gear with a relatively low cam angle for cooperation with gripping heads. This tong, which performs satisfactory for many applications, does not provide the minimum marking necessary for use on chrome plated oilfield tubulars.
The disadvantages of the prior art are overcome by the present invention, and an improved power tong as hereinafter disclosed, which is particularly suitable for taking up and breaking out chrome plated oilfield tubulars.